This application is submitted in response to NOT-OD-20-022 as an administrative supplement to NCATS CTSA Program KL2 Institutional Career Development Awards. The highly-successful University of North Carolina (UNC) Institutional Career Development Award KL2 program is housed within the UNC Translational and Clinical Sciences (TraCS) Institute and aims to shape the development of junior translational scientists such as the applicant of this proposal. The overarching goal of this proposal is to characterize longitudinal, brain-behavior trajectories, and developmental profiles from infancy through school-age in Down Syndrome (DS) compared to Autism Spectrum Disorder (ASD). The specific aims are to 1) characterize age-, disorder- and social communication (SC)-specific brain features in DS children from infancy through school age and 2) describe SC development during infancy and school age in DS children. This project is the first to address the lack of studies that 1) examine the onset and trajectory of SC symptoms in children with DS during infancy, 2) combine SC phenotyping with brain features in an early postnatal prospective, longitudinal sample and 3) compare brain biomarkers between with DS, iASD, and TD children both in infancy and childhood. This unparalleled opportunity to generate comprehensive trajectories of SC deficits in children with DS leverages data across four grants that are part of the Infant Brain Imaging Study (IBIS) Network making this large-scale, complex project feasible. This proposal addresses the goals of NCATS CTSA Program, the TraCS KL2 Program, and the INCLUDE Project in the following ways, 1) Funding this scholar?s project increases the translational workforce of DS researchers, 2) This project utilizes prospective, longitudinal cohorts of children with DS, ASD, and typical development (TD) thereby addressing the INCLUDE goal to investigate co-occurring conditions across the lifespan, and 3) integrates multiple fields in order to accelerate the development of new therapies and interventions for DS. Completion of this project will result in rich information about the development of SC and brain biomarkers for DS, ASD, and TD children with the aim of accelerating the development of new SC therapies and interventions for DS children.